This invention is directed to a catalyst system comprising a first-stage high temperature catalyst and a second-stage lower temperature catalyst, for removing nitric oxides (NO.sub.x), carbon monoxide (CO), and hydrocarbons (HC's) from exhaust streams of, e.g., lean-burn (less than stoichiometric amount of fuel) internal combustion engines.
A number of catalysts have been suggested to convert engine exhaust gas components like carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO.sub.x) into other gases. The first two are desirably oxidized to H.sub.2 O and CO.sub.2 while the nitrogen oxides present in the exhaust gas, generally nitric oxide, are desirably reduced to N.sub.2. These so called "three-way" catalysts achieve simultaneous efficient (conversion&gt; 80%) removal of CO, HC, and NO.sub.x when the fuel mixture of an internal combustion engine is slightly "rich" in fuel, i.e., in a narrow A/F ratio range between about 14.7 and 14.4, and the exhaust gas is slightly reducing. Such three-way catalysts are not efficient, however, in the reduction of NO.sub.x when engines are operated on the lean (reduced fuel) side where the A/F ratio is greater than 14.7, generally 19-27, and the exhaust gas is richer in oxygen. It is desirable, however, to operate engines on the lean side to realize a benefit in fuel economy, estimated to be in the range of 6-10%.
Considerable success has been achieved in the catalytic oxidation of unburned hydrocarbons and carbon monoxide, but the conversion of the nitrogen oxides has proven to be a much more difficult problem. This is because the reducing substances (such as CO or H.sub.2) tend to react more quickly with the free oxygen, O.sub.2, present in the exhaust gas than with the oxygen associated with nitrogen in NO.sub.x.
Fujitani el al., in U.S. Pat. No. 5,041,270, suggest a lean-burn catalyst system comprising a single support having a first copper catalyst for reducing NO.sub.x to N.sub.2 in the oxygen rich environment and a second oxidation catalyst comprising noble metals like Pd loaded on a porous support composed of, e.g., alumina for converting CO and HC. The use of the copper catalyst upstream of the noble metal oxidation catalyst is critical to this invention.
Subramanian et al., in application Ser. No. 07/772,410 filed Oct. 7, 1991 and entitled "Catalyst System for Converting Emissions of a Lean Burn Engine", similarly assigned as the present invention, describe a dual-bed catalyst system that comprises an acidic alumina catalyst (first stage) and a noble metal based oxidation catalyst (second stage).
The catalyst systems described have certain disadvantages. These include: a) relatively high cost associated with the use of precious metal catalysts, b) narrow operating temperature window because of the limited temperature range over which significant NO.sub.x removal is achieved, and c) either relatively low peak NO.sub.x conversion activity (acidic alumina) or poor durability (zeolite supported catlyst of Fujitani el al.).
It would be desirable to have a catalyst system particularly useful for lean-burn engines (oxygen rich exhaust situations) to effectively reduce NO.sub.x emissions and also provide high conversions for hydrocarbons and carbon monoxide. It would desirably allow for improved fuel economy. It would further be desirable if such a system would lower NO.sub.x emissions over a wider temperature range in comparison to other catalyst systems thereby offering more flexibility and convenience in determining catalyst location. If this system employed only base metals, it would additionally offer cost advantages over more expensive, conventionally used noble metal catalysts. The present invention provides such a system.